text: There are three classes of SMADs:
  # Receptor-regulated SMAD (R-SMAD)
  # Common-mediator SMAD (Co-SMAD)
  # Inhibitory SMAD (I-SMAD)
  Examples of SMADs in each class:
  The TGF-β superfamily activates members of the SMAD family, which function as transcription factors. Specifically, the type I receptor, activated by the type II receptor, phosphorylates R-SMADs that then bind to the co-SMAD, SMAD4. The R-SMAD/Co-SMAD forms a complex with importin and enters the nucleus, where they act as transcription factors and either up-regulate or down-regulate in the expression of a target gene.
  Specific TGF-β ligands will result in the activation of either the SMAD2/3 or the SMAD1/5 R-SMADs. For instance, when activin, Nodal, or TGF-β ligand binds to the receptors, the phosphorylated receptor complex can activate SMAD2 and SMAD3 through phosphorylation. However, when a BMP ligand binds to the receptors, the phosphorylated receptor complex activates SMAD1 and SMAD5. Then, the Smad2/3 or the Smad1/5 complexes form a dimer complex with SMAD4 and become transcription factors. Though there are many R-SMADs involved in the pathway, there is only one co-SMAD, SMAD4.